This invention relates to a vector control for an electric motor, and more particularly to a current detection method for an electric motor, which detects phase currents by single current detecting element, and a control apparatus for an electric motor using such a method.
In the case where vector control of an electric motor is carried out, it is required to convert an alternating current into an alternating voltage, and to obtain a voltage waveform having less phase lag with respect to a current waveform. As means for converting an alternating current into an alternating voltage, there are instrument current transformers generally called a CT (Current Transformer) and a Hall CT based on Hall effect, etc. Among them, the Hall CT is expensive and its use is limited. For this reason, a current detection method and a control apparatus for an electric motor using the instrument current transformer will be described below.
FIG. 8 is a block diagram showing the configuration of a conventional control apparatus for an electric motor, which detects a current by using a CT to carry out vector control of the electric motor on the basis of the detected value. In this figure, a smoothing capacitor 3 is connected to the output side of an AC/DC converter 2 also called a converter which converts an alternating current into a direct current of an AC power supply 1, and the smoothed direct current is delivered to a DC/AC converter 5 also called inverter. The DC/AC converter 5 is comprised of bridge-connected switching elements such as IGBT, etc., and serves to carry out ON/OFF control of these switching elements in accordance with a basic excitation pattern determined in advance to thereby convert a direct current into three phase alternating currents to deliver the three phase alternating currents to an electric motor 7. A current detecting resistor 4 for detecting an input current is connected to the DC side of this DC/AC converter 5, and CTs 6a, 6b for respectively detecting currents of two phases of three phase alternating currents are provided between the DC/AC converter 5 and the electric motor 7.
Moreover, an overcurrent detecting circuit 8 for detecting overcurrent is connected to the current detecting resistor 4, and a current detector 9 is connected to the CTs 6a, 6b. Among them, the current detector 9 detects currents of two phases on the basis of respective output signals of the CTs 6a, 6b, and detects, by calculation, current of the remaining one phase because a value obtained by totalizing instantaneous values of three phase alternating currents is equal to zero at all times to apply current detection signals of three phases to a rotor position detector 10. The rotor position detector 10 detects the rotor position of the electric motor 7 on the basis of the current detection signals of three phases.
Waveform generator 11 generates basic excitation pattern for carrying out ON/OFF control of plural switching elements constituting the DC/AC converter 5 in order to output pulse-width modulated three-phase alternating currents on the basis of the detected rotor position signal and a rotational frequency command given from the external. Further, a drive circuit 12 carries out ON/OFF control of the switching elements constituting the DC/AC converter 5 in accordance with this excitation pattern, and stops the ON/OFF control of the switching elements when the overcurrent detecting circuit 8 detects overcurrent.
In the above-described conventional control apparatus for an electric motor, in order to detect currents of three phases delivered to electric motor 7, two current detecting elements, i.e., CTs 6a and 6b were required. If there may be employed with respect to such a configuration an approach in which the number of current detecting elements is reduced to one and the current detecting element is removed from the connecting path of electric motor 7 with respect to DC/AC converter 5, simplification of the configuration including wiring can be realized. This is advantageous.
This invention has been made in view of the above circumstances, and its object is to provide a current detection method for an electric motor, which detects currents of respective phases delivered to the electric motor by single electric detecting element, and a control apparatus for an electric motor using such a method.